The current proposal is aimed at understanding the neurobiological mechanisms underlying anxiety disorders. Furthering the understanding of anxiety disorders remains a key unmet public health goal, as these disorders represent a large burden to society in morbidity and related costs. Our previous studies with wild-type and serotonin 1A- receptor knockout (5-HT1AR KO) mice have implicated a circuit consisting of the ventral hippocampus (vHPC), the basolateral amygdala (BLA), and the medial prefrontal cortex (mPFC) in regulating anxiety-like behavior in various paradigms. Most notably, we have shown that these three regions functionally interact during anxiety, and that the strength of these interactions (1) correlates with anxiety and (2) is greater in the highly anxious 5- HT1AR KO mice than in wild-type littermates. Furthermore, our data suggest that the mPFC uses these interactions to construct a map of the aversiveness of the environment, which in turn is used to guide behavior. However, these observations are primarily correlative in nature. In this renewal application, we propose to directly test the hypothesis that the vHPC and BLA drive neural representations of aversiveness and anxiety-related behavior via their interactions with each other and with the mPFC. We will use a variety of pharmacological, optogenetic and pharmacogenetic tools to silence the vHPC and/or BLA, combined with in vivo neurophysiology to probe the role of these regions in the neural representations of aversiveness within the mPFC as well as anxiety-like behavior. In parallel, we will use emerging technologies aimed at silencing specific projections from and between these brain regions to determine whether and how these specific projections are required. These experiments will be performed in both wild-type and 5-HT1AR KO mice to test their relevance to pathological as well as physiological anxiety mechanisms. Addressing these issues will clarify the mechanisms by which these brain structures modulate anxiety-like behavior, and may help identify specific patterns of connectivity and activity that underly anxiety, providing novel functional targets for therapeutic intervention.